Hearing aid performance continues to be enhanced by technological advances in audio signal processing capabilities, e.g. primarily via digital signal processing, environmental adaptation, programmability, and noise cancellation techniques. As part of these on-going advancement efforts, hearing aid manufacturers often request performance improvements from suppliers of microphone transducers.
One performance limitation of today's assisted-listening systems and/or devices, e.g., hearing aid—often associated with noise canceling directional hearing aids utilizing high performing miniature microphones—is caused by the substantial reduction of the impedance of the anti-parallel diode biasing circuitry commonly implemented at the input of the microphone buffer. The anti-parallel diode biasing circuitry coupled to the signal input of the microphone will effectively limit the transducer output to the turn-on voltage, Von, of the diodes, e.g., +/−0.3V, relative to the zero-bias potential.
Although diode biasing has been a major factor leading to significantly improved microphone noise performance, it has been noted that diode biasing also results in undesirable audible artifacts and performance degradation in directional hearing aids. Frequently, such performance degradation occurs under loud acoustic transient conditions, i.e., when the signal level voltage from the transducer would normally exceed Von.
Thus, an alternative input biasing configuration is desirable to adapt the microphone buffer to accommodate environmental conditions for alleviating these audible artifacts without compromising overall noise performance in the transducer.